Introduction
to LANDSAT

The LANDSAT Program
is the longest running exercise in the collection of multispectral, digital
data of the earth's surface from space. The program has operated continuously
since LANDSAT 1 (then the Earth Resources Technology Satellite (ERTS)
1) was launched on July 23, 1972. More than 3 million images from the
Multispectral Scanner System (MSS) on LANDSATs 1-5 have been aquired and
stored at the National Satellite Land Remote Sensing Data Archive (NSLRSDA)
at EROS Data Centre (EDC), Sioux Falls, South Dakota and the LANDSAT international
ground stations. The temporal extent of the collection, the characteristics
and quality of LANDSAT data, and the ability to collect new data directly
comparable to that in the archive, make LANDSAT data a unique resource,
one used extensively to address a broad range of issues in earth science,
global change science, and making monitoring and assessing land and coastal
zone resources.

Landsat
Platforms

Five LANDSAT satellites
have been successfully launched commencing with LANDSAT 1 in July 1972.
All 5 satellites have operated from a repetitive, circular, sun-synchronous,
near-polar orbit and on each day-side pass, scan a ground swath 185km
wide beneath the satellite. The first three satellites carried the Multispectral
Scanner (MSS) as the main imaging instrument with a Return Beam Vidicon
(RBV) as a subsiduary. The paths of these satellites were inclined 99
degrees with an 18 day repeat cycle and an equatorial crossing of between
8:50 and 9:30am local time. The last two satellites (LANDSATs 4-5) had
the Thematic Mapper (TM) sensor as well as the MSS, are inclined 98 degrees,
have a repeat cycle of 16 days and have an equatorial crossing of 9:45am
local time. The nominal altitude of the satellites was 920km for LANDSATs
1-3 and 705km for LANDSATs 4-5.

The RBV system
on LANDSAT 1-2 consisted of three cameras which took simultaneous
images of the earth in different spectral bands. The area covered
was similar to the MSS images. The images were stored on the RBV photosensitive
surfaces from which they were scanned to produce video outputs. The
RBV system on LANDSAT 3 consisted of two identical side-by-side
instruments which scanned in one spectral band. Each camera produced
an image equivalent to a quarter of an MSS scene and the resolution
was approximately 40 metres.

LANDSAT 6 was
to represent a departure from the earlier satellites with an enhanced
Thematic Mapper and no MSS. Unfortunately this satellite, launched
in early 1993, was lost on launch without any backup. LANDSAT 5 is
currently the only satellite routinely providing Thematic Mapper and
MSS data on direct downlink to ground stations. Although the satellite
operators are maintaining an optimistic outlook for LANDSAT 5, suggesting
that it has sufficient fuel to last the turn of the century, some
degradation of the imagery has occurred and this might be expected
to worsen.

The next satellite
in the LANDSAT series will be LANDSAT 7 and is likely to be a joint
project between NASA, NOAA and the USGS. The Department of Defence
has apparently pulled out of the LANDSAT Program Management, after
funding could not be obtained for a second high resolution sensor
on the satellite. The prime instrument on board the satellite will
be the Enhanced Thematic Mapper (ETM+). This instrument has a similar
7 bands multispectral capability as the LANDSATs 4-5 but with a 15
metre resolution panchromatic band (0.5 to 0.9 micrometres, visible
green to near IR), which is co-registered with the multispectral data.
The ETM+ will be capable of collecting 250 day-lit, land mass scenes
per day and transmitting these scenes to ground stations via a 150Mbps
X-band link. The spacecraft will also have 375GB of solid state memory,
sufficient for 100 ETM+ scenes, for delayed transmission.

Landsat 7 is not
expected to be launched until 1998.

Data
Reception

When within line
of sight of a ground receiving station, data from LANDSATs 1-3 were
transmitted directly to the station and recorded on magnetic tape.
These satellites also had onboard tape recorders so that data from
the MSS or RBV could be recorded and transmitted to a ground station
when passing over the US. Availability of worldwide MSS data outside
the limits of ground reception facilities is relatively good and is
available from EROS.

LANDSATs 4-5 did
not have onboard tape recorders and relied on direct transmission
to ground receiving stations and on transmission via the Tracking
and Data Relay Satellite System (TDRSS) to the White Sands ground
station in the US. The TDRSS satellites are in geosynchronous orbits
and if fully operational (using satellites designated as east and
west) allow the acquisition of data for all the earth's surface except
for an area within 50 deg north and south and 67 to 82 degrees east.
The TDRS-1 (east) satellite operates at 41 degrees west longitude
over the Atlantic Ocean off the north-east coast of Brazil. The launch
failure of the companion TDRSS satellite during the Challenger explosion
in January 1986 limited the availability of MSS and TM from these
satellites over Asia and the Pacific region to direct reception. A
replacement for this satellite was in place in late 1988 at 171 degrees
west longitude slightly south-west of Hawaii. The TDRSS link on Landsat
5 is no longer operational.

Availability of
the TDRSS has meant that worldwide MSS and TM from LANDSATs 4-5 outside
the limits of direct ground reception faclities is good for the Americas,
Africa and Asia west of 67 degrees east. For Asia, east of 82 degrees,
availability of data is limited to a few years between 1989 and the
early 1990's.

Data from the
LANDSAT satellites is collected in a continuous stream of data along
a near vertical path as the satellite moves from north to south. The
data is arbitarily divided into nominal scenes which are about 24
second increments of spacecraft time apart, corresponding to a spacing
of approximately 160km. This path/row designation is referred to
as the LANDSAT Worldwide Reference System (WRS). The rows have been
positioned in such a way that Row 60 coincides with the equator. This
reference system is different between LANDSATs 1-3 and LANDSATs 4-5
because of the different altitudes of the satellites. This effects
the spacing of the paths with LANDSATs 1-3 having 251 paths worldwide
and LANDSATs 4-5 having 233. Designation of the rows is similar along
each path. A series of WRS path/row maps is available world wide from
EOSAT and these normally have the LANDSATs 1-3 WRS on one side and
the LANDSATs 4-5 on the reverse. The map sheets are at 10 million
scale and 26 are needed for global coverage.

LANDSAT Path designations
increase from east to west while SPOT are the reverse.

The standard MSS
and TM scenes from each of the LANDSAT satellites have had the same
nominal ground coverage of 185km by 170km. Since the LANDSATs 1-3
had 251 paths compared to the 233 of LANDSATs 4-5, there was considerably
more overlap between adjacent paths on the earlier satellites. This
overlap is shown in the following table.

It is possible to view the images stereoscopically in this overlap
region and at latitudes greater than 60 degrees there is complete
stereoscopic coverage. The quality of the viewing will be dependent
on the relief in the overlap area with the angle of the capture varying
from zero in the centre of the scene to a maximum of 8 degrees at
the extreme edges of the scenes.

LANDSAT
Ownership

The LANDSAT Program
during the 1970's was run by NASA as a high-tech experiment. At the
end of 1979, control of the program was passed to the Department of
Commerce's National Oceanic and Atmospheric Administration (NOAA)
and plans were announced for a phased transition to commercialization
of the operation of the program. In 1984, the US Congress passed the
LANDSAT Remote Sensing Commercialization Act (P.L 98-365) which officially
authorised moving the LANDSAT program to the private sector. The tendering
process for the commercial operator was won by the Earth Observation
Satellite Company (EOSAT) which was jointly owned by General Electric
Co. and Hughes Aircraft Co.. The contract included operating LANDSATs
4-5 and designing , developing , launching, and operating the follow-on
system.

In October 1992,
the Land Remote Sensing Policy Act (P.L. 102-555) was passed effectively
reversing the 1984 decision to comercialise the LANDSAT system. This
recognised the failure of the commercialization process because the
commercial price of data hindering distribution. The Act committed
the US to maintain continuity of LANDSAT-type data into the next century
and commited funding to NASA and the Department of Defence for the
procurement of LANDSAT 7.

In November 1990,
EOSAT and NOAA reached an agreement which allowed MSS data older than
two years to be sold at the EROS Data Centre (EDC) for the cost reproduction
and distribution. This agreement was broadened in 1993 and EOSAT discontinued
marketing all MSS data effective Feburary, 1993. This agreement now
means that all archived MSS data is available from EDC at relatively
low prices.